In satellite communication systems, information is being transmitted from one earth station to another via a satellite. In order to transmit and receive information, each earth station may be equipped with at least a dish antenna and a satellite transmitter (referred to herein as block up converter, or BUC). The ability of one earth station to receive transmissions of another earth station via a satellite may depend on several factors, including the gain of the transmitting and receiving antennas, the output power of the transmitting BUC, one or more parameters of the satellite being used (e.g. gain) and the positions of the earth stations relative to the radiation foot-print of the satellite.
Another important factor which may affect reception capability is related to atmospheric conditions, particularly the existence of water in the signal's path at the time of transmission. The more water in the signal's path (e.g. in the form of clouds, vapors, rain, hale or snow), the higher the attenuation inflicted by the atmosphere upon the transmitted signal. Since a signal transmitted over satellite fades as rain intensity increases, this entire phenomenon is often referred to as rain fade (i.e. regardless if the attenuation is caused by rain or another form of showers e.g. snow). The effects of rain fade (i.e. the magnitude of the attenuation inflicted on a transmitted signal) depend on the frequency band being used (e.g. C-band, Ku-band, Ka-band or another), where the higher the frequency the deeper the fade being experienced for a given level of rain intensity.
Rain fades may affect the availability of a satellite communication network. If various gains are insufficient and/or if transmission power level is not high enough to begin with, connectivity between earth stations may be interrupted during rain fades.
One method for coping with rain fades and insuring high availability is using high gain antennas and high power BUC units. However, since the gain of a dish antenna is proportional to its size (i.e. its diameter), a high gain antenna also means a large antenna, which may require special installation considerations, motorized pointing and tracking systems, etc. Thus large antennas, as well as high power BUC units, are quite expensive. Therefore, this method is very expensive and in some cases may be economically infeasible.
On the other hand, the total duration of extreme rain fades is usually measured in several hours per year. Therefore, users of satellite communication systems are often willing to accept loss of connectivity during such times in order to use less expensive equipment and to make such networks economically feasible. For example, some satellite communication users may accept an availability level of 99.8%, which corresponds to total unavailability of approximately 17.5 hours per year. Some users may require higher availability and some will settle for lower availability (e.g. 99.5% which corresponds to slightly less than 2 days of outage per year).
The introduction of adaptive coding and modulation techniques (e.g. in the 2nd generation Digital Video Broadcasting via Satellite standard (DVB-S2)) may enable satellite communication systems to be designed in a more cost-effective manner. A network may be designed to perform at high efficiency at clear-sky (i.e. best) conditions and to switch to less efficient but more robust modulation and coding whenever degradation in link conditions is experienced. During degradation periods, the total throughput over a satellite link may be reduced but communication may still be available.
In a satellite communication network, comprised of a central hub and a plurality of remote terminals, information in the inbound direction (i.e. from the remote terminals to the central hub) is often transmitted over shared bandwidth for at least the purpose of increasing bandwidth utilization efficiency. There are many methods known in the art, also known as access schemes, for administrating use of the common inbound bandwidth. Of these methods, those based on reservation techniques (i.e. where a remote terminal is allowed to transmit only on bandwidth assigned to it, e.g. by a central hub) have the highest bandwidth utilization efficiency.
However, all known reservation techniques are based on a predefined time-frequency plan (TFP). Both the hub and the remote terminals are aware of a division of the inbound bandwidth to channels and possibly of each channel to timeslots. Channel speed (i.e. symbol rate) as well as modulation and coding of each timeslot may be predefined. Therefore, use of adaptive modulation and coding techniques in the inbound direction of said satellite communication networks is quite limited.
Furthermore, in order to achieve bandwidth utilization efficiency during at least 99% of the time, most of the inbound channels should be designed for clear-sky conditions, during which the expected reception signal to noise ratio (SNR) is sufficient to support higher spectral density (i.e. more data bits per each bandwidth unit). However, such design is vulnerable to rain fade events, which may affect the link between a central hub and the satellite, especially if the antenna at the hub is relatively small and/or the satellite being used has relatively low gain. During such events, most of the inbound bandwidth becomes unusable as the reception SNR drops below the level necessary for receiving a signal transmitted using the predefined channel speed, modulation and coding. Consequently, most if not all the remote terminals in such networks experience lack of service, which may last as long as fade conditions persist, sometimes for many minutes.